<script>
  Promise.resolve().then(() => {
    // #1
    Promise.resolve().then(() => {
      // #2
      console.log('f2')
    }).then(() => {
      // #4
      console.log('9')
    }).then(() => {
      console.log('3')
    })
  }).then(() => {
    // #3
    console.log('56')
    Promise.resolve().then(r => {
      // #5
      console.log(r)
    })
    // !我说默认会有一行下面的返回值，但是不要明确写，如果明确些，含义完全不一样，相当于会再次嵌套一层 Promise
    // return Promise.resolve(undefined)
  }).then(() => {
    // #6
    console.log('22')
  });
</script>